Filtration is well known for the treatment of fluids, such as water, and is typically achieved by use of appropriate filtration media, such as filtration membranes. A conventional arrangement for housing and utilising filtration membranes is to arrange each membrane unit (hereafter called a “module”) into its own pressure containing housing which has suitable connections to allow filtration and cleaning cycles to be performed on the membrane. Although such modules may be provided in numerous sizes, for illustrative purposes a larger module may be around 250 mm in diameter, around 2000 mm long, and may contain around 100 m2 of membrane surface. A typical operation flux may be 60 LMH (litres per hour per m2 of membrane surface) which means the flow of a large module may be 6 m3/h. Where larger flows must be filtered these modules are usually connected to manifolds so that many modules are operated in parallel. Typical applications where large flows must be treated may include filtering seawater or brackish water prior to a reverse osmosis process to produce fresh drinking water or process water, and filtering seawater prior to a nanofiltration process used to remove particular ionic species from the water prior to injection into oil reservoirs. These applications may require flows of up to 1000-3000 m3/h which would require 150 to 450 of these large modules, and would require an even larger number of smaller modules.
It has been proposed in the art to include multiple membrane modules within a common filtration vessel or tank, wherein a single feed of raw fluid is delivered to the vessel which may then be treated by all modules within the vessel. This may minimise infrastructure, such as individual module pressure housings, manifolds, valves and the like, and may minimise the footprint of the filtration system.
U.S. Pat. Nos. 5,209,852 and 7,083,726 each disclose a filtration vessel containing multiple membrane modules suspended from a partition plate, wherein the partition plate divides the vessel into lower and upper compartments. During use, the lower compartment receives and contains raw water at the required filtration pressure, and the upper compartment accommodates filtered water which has passed from the lower compartment through the individual membrane modules. The partition plate must isolate the upper and lower chambers and be of sufficient integrity to accommodate the pressure differential therebetween.
In known filtration vessels with multiple modules, such as disclosed in the prior art documents mentioned above, each module is mounted in respective holes formed in the partition plate, wherein the holes accommodate the full outer diameter or width of the modules. Accordingly, the size and number of the holes can significantly weaken the plate and as a result partition plates are typically formed to be relatively thick, which can increase the cost of the vessels, particularly where expensive materials, such as titanium, are required, for example to resist corrosive chemicals which may be present in the vessel.
Furthermore, as the individual holes in a partition plate are dimensioned to accommodate the full width of the filtration modules, the required sealing area to maintain isolation between upper and lower chambers can also be relatively large, increasing the potential risk of leakage between chambers.
The efficiency of filtration media such as membranes will reduce over time due to fouling, which typically results in an increase in the pressure drop across the media. Such fouling is addressed by cleansing processes, usually on a cyclical basis, to maintain efficient operation.
In a typical application of filtering seawater using membrane media, a membrane may need to be frequently cleaned, for example every 30 to 90 minutes to maintain its filtration capacity. It has been found that a quick clean, for example of around 1 to 3 minutes involving only physical cleaning mechanisms can be effective in recovering most of the pressure drop increase which has occurred. This clean can be referred to as a type 1 clean. However this type 1 clean does not fully clean the membrane and as such there is a slow increase in the pressure drop across the “cleaned” membrane.
After typically 18 to 48 hours a longer clean involving chemical cleaning mechanisms is generally employed to recover the “cleaned” membrane pressure drop that the type 1 clean may not be capable of recovering, and/or to disinfect the membrane to prevent growth of bacteria which can also foul the membrane. This clean can be referred to as a type 2 clean.
A type 2 clean, however, typically still does not fully clean the membrane, such that over a longer period of perhaps 2 weeks to 2 months another type of cleaning of greater thoroughness and even longer duration or cost is required. This clean can be referred to as a type 3 clean
A type 1 clean tends to use physical cleaning mechanisms which can be achieved by the operation of valves that cause changes in flowrate, flow direction, or fluids in the module.
Type 2 and 3 cleaning are generally similar to each other and typically include the use of chemicals. However, a type 2 clean usually employs fewer steps and is of shorter duration than a type 3 clean, such that the membrane modules are out of service for less time.
It is common for a type 1 clean to be performed before and/or after a type 2/3 clean.
Although many cleaning processes exist, it is often the case that a filtration apparatus is only capable of supporting a very limited range of these. For example, in known filtration apparatus which include multiple modules suspended from a partition plate in a vessel, it is generally not possible to flow or wash through the modules in reverse directions simultaneously, for example to perform both back and forward washing. This is because the lower chamber would contain both the water to be introduced into a feed side of the modules, and also the dirty water which has been backwashed through the modules.
Furthermore, certain cleaning operations may utilise the bubbling of a gas through the modules to agitate or scrub the filtration media and dislodge particulate and other matter. In known vessel based systems gas nozzles are located within the lower chamber generally below a respective filtration module, such that gas exiting the nozzles rises towards, into and through each module. However, as the modules will require to be filled with water to permit the bubbles to have the desired effect, then the lower chamber must be filled, and it is likely that a degree of turbulence within the lower chamber will exist. This may disturb the gas exiting the nozzles which may result in an uneven distribution of gas into the modules.
Further, in the known vessel based systems with multiple modules, cleaning times may be extended in that for many cleaning processes the entire lower chamber will need to be drained.